Oftentimes it is required to expand the coverage of local area networks (LANs) by interconnecting these LANs to create a geographically-disperse metropolitan area network. Also, an organization such as a university or a company that operates a distinct set of LANs, say within a building complex, may find it necessary to interconnect these LANs to effect a large LAN serving the entire complex.
If a device (e.g., computer, video terminal, telephone set) connected to one LAN has only one path to a device on another LAN after interconnection, thereby exhibiting a loop-free topology, interconnection may be achieved by connecting pairs of networks with a so-called gateway that executes a store-and-forward protocol. Such an interconnection arrangement is described as a transparent interconnection since the existence of the gateway is not visible to any of the devices in the linked system and, consequently, no modifications to the devices or the messages or packets propagated by the devices are required.
Recently, a number of references have discussed the methodology and associated circuitry for the transparent interconnection arrangement. These include: (1) "Local Area Network Applications", Telecommunications, September, 1984 by B. Hawe and B. Stewart; (2) "An Architecture for Transparently Interconnecting IEEE 802 Local Area Networks", a Digital Equipment Corporation Technical paper submitted to the IEEE 802 Standards Committee in October, 1984; and (3) "Transparent Interconnection of Local Networks with Bridges", Journal of Telecommunications Networks, October, 1984 by B. Hawe, A. Kirby and B. Stewart. These references stress that the physical topology of the interconnected networks must be that of a branching tree. Gateways cannot transparently interconnect local area networks that support alternative paths between local networks resulting in loops. In fact, in reference (1), a technique is suggested for transforming a general mesh topology to a loop-free topology so that the gateways may be utilized.
The requirement that the system topology be loop-free is a severe one in general, and ultimately restricts the practical application of the conventional gateway arrangement. In order to satisfy channel capacity demands or to provide a degree of reliability, an interconnected system will contain some loops in some portions of the topology. The conventional gateways always detect and remove these loops, preventing any improved redundancy or reliability. The problem of interconnecting loop or cyclic topologies at the physical or link layer has not been addressed by prior art references.